1. Field of the Invention
The present invention relates to hydrostatic transaxles intended primarily for use in the lawn and garden industry on riding lawnmowers, lawn and garden tractors and the like, but may also be applied to larger implements and vehicles.
2. Description of the Related Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically oil, and then back to rotary mechanical motion to rotate a pair of drive axles in order to drive the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of the radial piston design, for example, and motor formed as a gear pump. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring or swash plate.
It is well known to provide a xe2x80x9cneutral switchxe2x80x9d for use with many vehicle types so that an operator is prevented from starting or activating the vehicle when the vehicle""s transmission is engaged. Neutral switch applications, heretofore, include neutral switch placement, specifically the motion sensing portion thereof, external to a vehicle""s transmission to monitor the neutral position secondarily through linkage positioned remotely respective of the pump. Problems arise when linkage becomes loose or worn or if the switch is damaged through the normal rigors of agricultural usage. One such problem involves xe2x80x9ccreepxe2x80x9d i.e., slight movement of the vehicle when apparently in neutral, due to loose linkage between a shift lever and the internal componentry comprising the transmission. Often, the neutral switch contacts or registers off of loose linkage which allows activation of the power source even though the transmission remains engaged, albeit slightly.
In response to difficulties encountered with neutral switch placement exterior to the transmission, the neutral switch, or the motion sensing portion thereof, was installed internally for use with radial piston-type hydrostatic transmissions. Although the benefits of the neutral switch may be appreciated with radial piston hydrostatic transmissions, axial piston hydrostatic transmissions include certain advantages over their radial counterparts. One such advantage is reduction in overall transmission size which provides for a reduction in materials corresponding to a decrease in cost. A neutral switch adaptable to an axial piston hydrostatic transmission, capable of registering a neutral position internally to the transmission would be most desirable. Another problem incident with hydrostatic transmissions, specifically neutral arrangements and adjustability assemblies therefor, is the lack of a neutral adjustment assembly, externally accessible and low in cost.
The speed of a hydrostatic transmission is generally selectively controlled by an operator via a hand control or foot pedal control, for example, by varying the eccentricity of the pump track ring or swash plate. Hydrostatic transmissions do not always provide a true xe2x80x98neutralxe2x80x99 when first assembled (i.e., transmission fluid is pressurized by the pump, albeit minimally, which can cause rotation of the axle). Manufacturers confront this problem by providing a neutral adjustment mechanism, as part of the hydrostatic transmission, which cooperates with the swash plate such that a true neutral may be exacted after assembly. Some neutral adjustment mechanisms are provided internally within the hydrostatic transmission casing. Included in the group of neutral adjustment mechanisms, customarily used, is the extendible threaded linkage shaft. The linkage shaft allows an operator fine adjustment of the swash plate by extending or collapsing, via threaded engagement, the threaded shaft to effectuate the adjustment.
The neutral adjustment offered by a threaded shaft tends to vibrate loose, and additionally, fine adjusting via a threaded shaft is often a cumbersome task since the shaft must be secured and the ends thereof extended. Internal type neutral adjustment mechanisms are also cumbersome since substantial disassembly of a vehicle is generally necessary to access the neutral adjustment device. What would be highly desirable is a neutral adjustment mechanism that is accessible subsequent to assembly of a vehicle and one which preserves a neutral setting once adjustment is finalized. Further, a neutral adjustment assembly which readily adapts to the structure of the existing neutral return assembly structure, mandating few additional parts and little if any additional machining, would be highly desirable.
The present invention overcomes the disadvantages of the prior art by providing a hydrostatic transaxle including a neutral switch, disposed within a transmission casing and registering off of the swash plate. Moreover, a neutral adjustment assembly is provided, to overcome further disadvantages plagued by the prior art, by providing a neutral adjustment assembly including an externally arranged two-piece shift lever structure facilitating neutral adjustment externally and within in proximity to the shift lever, which is particularly useful in foot pedal control applications.
The hydrostatic transaxle of the present invention is drivingly connected to a power source which is electrically activated through an ignition circuit and a variable displacement pump is fixed relative to the casing and driven by the power source. The pump includes a pump cylinder barrel which includes a plurality of parallel arranged axially reciprocable pistons. A swash plate is positioned between the pistons and the casing and includes a lateral surface rotatably engaged within a recess defined by the casing. Each piston is engaged with the swash plate whereby tilting of the swash plate effectuates a fluid displacement in the pump. A neutral switch includes a portion fixed to the casing and a registering portion internally positioned within the casing. The registering portion of the switch is positioned against the swash plate itself whereby pump displacement electrically deactivates the ignition circuit.
The present invention further provides a neutral adjustment assembly for use with a hydrostatic transmission including a casing enclosing the hydrostatic transmission and a control rod rotatably supported by said casing and including a portion positioned externally relative to said casing. A two member shift lever is provided which includes a first member attached to the externally positioned portion of the control rod and a second member adjustably fixed relative to the first member. A resilient member, having a first end held to the casing and a second end held to the second member of the shift lever wherein the resilient member urges the first member of the shift lever into a neutral position. At least one fastener is provided and releasably secures the first and second members together, wherein the first member is moveable relative to the second member through at least one slot formed in either the first or the second members of the shift lever.